Q: My imac tells me i have to restart my computer

It's called a kernel panic.

What is a Kernel Panic?

http://support.apple.com/kb/TS3742

and  http://support.apple.com/kb/TS1892?viewlocale=en_US

http://developer.apple.com/mac/library/technotes/tn2002/tn2063.html

More details here:

http://www.thexlab.com/faqs/kernelpanics.html

Useful article on how to avoid and eliminate kernel panics here:

http://reviews.cnet.com/8301-13727_7-10344626-263.html?tag=mfiredir

Can you post some of your crash logs? Might be a clue. See Mac OS X: How to log a kernel panic http://support.apple.com/kb/HT2546

Understanding crash logs isn’t easy and it’s hard (sometimes impossible) to decipher the cause of the problem. Take a look at Apple’s Crash Reporter document at http://developer.apple.com/technotes/tn2004/tn2123.html Also look at Tutorial: An introduction to reading Mac OS X crash reports

http://www.macfixit.com/article.php?story=20060309075929717

Kernel panics are usually caused by a hardware problem – frequently RAM, a USB device or a Firewire device. What external devices do you have connected? When trying to troubleshoot problems, disconnect all external devices except your monitor, keyboard and mouse. Do you experience the same problems?

To eliminate RAM being the problem, Look at this link: Testing RAM @ http://guides.macrumors.com/Testing_RAM Then download & use Memtest & Ramber.

Do you have an Apple Hardware Test disc (the AHT is on the Install/Restore DVD that came with your Mac)? Running the Apple Hardware Test in Loop Mode is an excellent troubleshooting step for finding intermittent hardware problems. It is especially useful when troubleshooting intermittent kernel panics. If Loop Mode is supported by the version of the Apple Hardware Test you are using, you run the Extended Test in Loop Mode by pressing Control-L before starting the test. Looping On should appear in the right window. Then click the Extended Test button.The test will run continuously until a problem is found. If a problem is found, the test will cease to loop, indicating the problem it found. If the test fails, be sure to write down the exact message associated with the failure.In some cases, RAM problems did not show up until nearly 40 loops, so give it a good run.

http://macosg.com/group/viewtopic.php?t=800 12-Step Program to Isolate Freezes and/or Kernel Panics

 Cheers, Tom